Interpretive Summary: Scientists are currently unable to fully measure the exchanges of energy at the Earth's surface during freezing and thawing events. In particular, there is no existing method to measure the amount of energy flowing into or out of the soil as it freezes and thaws. The difficulty lies in the fact that the thermal properties of soil change dramatically over very small temperature ranges near the freezing point. We have developed a method that will permit soil thermal properties and energy flow to be accurately monitored during freezing and thawing events. This will benefit scientists who will apply the method to gain greater understanding of how Earth's weather and climate are influenced by freezing and thawing of the land surface.

Technical Abstract:
When soil freezes or thaws, latent heat fluxes occur and conventional methods for monitoring soil heat flux are inaccurate, often wildly so. This prevents the forcing of surface energy balance closure that is used in Bowen ratio flux measurements and the assessment of closure that is used as a check on the accuracy of eddy covariance measurements. We hypothesize that heat pulse sensors can be used to obtain accurate measurements of apparent thermal conductivity (Ka) and apparent volumetric heat capacity (Ca) which, together with soil temperature data, will permit accurate monitoring of soil heat flux under freezing and thawing conditions. Wintertime apparent thermal properties were monitored under soybean [Glycine max (L.) Merr.] residue using heat pulse sensors and independently predicted using a theoretical model. The measurements and the model were in agreement. Both showed that for temperatures between -5 and 0 Celsius Ka and Ca are strongly temperature dependent, varying up to three orders of magnitude. This temperature dependence is primarily the result of latent heat transfer processes. Measured soil heat flux during spring thaw ranged from -200 to 200 W m^-2. During soil thawing and snowmelt infiltration, we measured a latent heat flux into the soil of 9.6 MJ m^-2 over seven days, a sizeable heat flux completely undetectable by previous methods. The results of this study support our hypothesis. Further evaluation and application of this approach in the context of wintertime surface energy balance studies is warranted.